This invention relates to the manufacture of fibrous, flat-shaped articles and, more particularly, to a process for coagulating aqueous heat-coagulatable polymer binder dispersions impregnated in or coated on such articles.
The term "flat-shaped articles", as used in this application, includes material formed from individual fibers such as, for example, paper, woven fabric, knitted fabric and non-woven fabric. Of these, since non-woven fabrics are the most interesting, with their broad field of use, the invention will be described using the term non-woven fabric. Nonetheless, it should be clear that the inventive process is also applicable to the above-mentioned broader class of fibrous flat-shaped articles.
It is known to make non-woven fabric by forming a non-woven sheet, impregnating the sheet with an aqueous dispersion of a heat-coagulatable polymer binder and to coagulate the dispersion by heating (see U.S. Pat. No. 3,776,799). Known techniques for accomplishing such heating include the use of hot air, heated drums and infrared radiation. However, when a non-woven sheet is heated on both sides by any one of these techniques, water evaporates from both surfaces and the binder migrates toward the evaporation surfaces leaving a space in the middle of the sheet which contains only a very small proportion of binder. Such uneven distribution of the binder in the sheet tends to weaken it and causes it to split or delaminate along the core (i.e., in the area of low binder concentration) when subjected to severe stress. Binder migration upon drying, therefore, presents the man skilled in the art with serious problems.
In order to eliminate these problems, binder dispersions which reduce or eliminate binder migration on exposure to heat have been developed. One known technique to reduce binder migration is to add a thickening agent to the dispersion. Thickening agents, however, are difficult to remove from the sheet after the sheet has been dried and if they remain in the sheet they tend to impair the hand of the finished fabric.
A better known solution to the problem of binder migration is the use of heat sensitizing agents in the dispersion which causes the binder to begin coagulating at such low temperatures that pratically no evaporation takes place before coagulation begins. Depending on the heat sensitizing agents used and the binder system with which they are used, coagulation takes place either by the dispersed binder particles agglomerating to form larger particles which adhere to the fibers in the sheet, or the dispersion, with the binder in it, solidifies into a gel. In either case, by choosing one of these systems and appropriate choice of and proportions of chemical agents, the dispersion can be adjusted to be stable up to a desired temperature below 100.degree.C. and to coagulate suddenly, for example, at between about 30.degree.C. and 80.degree.C. When a non-woven sheet is impregnated with such heat sensitized dispersions and the sheet is subjected to heat, the effect of the heat is to cause sudden coagulation of the binder in the dispersion either by flocculation onto the fibers or by the formation of the dispersion into a gel. In both cases, the location of the binder throughout the fabric in the heating zone is fixed at a temperature below the evaporation temperature of the water so no migration can occur even during subsequent drying of the sheet.
Though these techniques work well in theory, in actual practice, various difficulties have been encountered. Thus, heretofore, for example, after a non-woven sheet has been impregnated either by spraying or dipping, etc., it has in some instances been heated by direct contact with steam heated drums or rolls such as can dryers. While such dryers heat the material sufficiently for coagulation to occur, the binder tends to migrate in a direction away from the drum and the material tends to stick to the dryers, causing surface defects in the sheet.
Infrared heating, while avoiding the surface defect problem of steam heated drums, has several other problems associated with it. The energy density of infrared radiators is low so the area which they cover has to be relatively large. Further, when a continuous web of heat-sensitized aqueous binder dispersion impregnated non-woven material is passed horizontally between infrared radiators and the web breaks, it falls on the lower radiator, presenting a fire hazard. Also, continued processing of the web is delayed while the radiators cool sufficiently to remove it and rethread the apparatus.
Further, it is difficult to adjust the heat output of infrared radiators so that coagulation occurs without unwanted drying. If, on the other hand, a coagulation by flocculation heat-sensitized aqueous binder system is used and the impregnated fabric passes vertically between two infrared radiators, the aqueous residue which is liberated on coagulation of the binder dispersion tends to flow downward in the sheet from the point of coagulation making it impossible to control the dilution of the binder in the non-coagulated portion of the sheet resulting in a finished product having non-uniform properties. The flow of this residue to still uncoagulated portions of the material not only dilutes the binder dispersion in those portions, it also means that more heat energy has to be imparted to that area to achieve coagulation. If this heat is not forthcoming, the material will contain binder in uncoagulated form and migration problems will occur during drying of the sheet at a later stage.
Accordingly, it is the principal object of this invention to provide a method which avoids the disadvantages of known prior art techniques and in which binder migration is prevented in a simple, economic manner.